Pollutant decomposition device

ABSTRACT

Pollutant decomposition device, including at least one outer transparent sheet and at least one inner transparent sheet being arranged such that a gap is formed between them and such that the gap is in communication with a surrounding gaseous composition on one side of the device such that the gaseous composition can pass through the gap. The device further including a photocatalyst arranged in the gap for depolluting the gaseous composition that pass through the gap. To obtain optimum decomposition efficiency the outer transparent sheet has a high degree of ultraviolet transmittance compared with the inner transparent sheet.

THE FIELD OF THE INVENTION

The present invention relates to a pollutant decomposition device,capable of decomposing pollutants by photocatalytic reactions.

BACKGROUND OF THE INVENTION

Pollution of air inside buildings or other confined spaces often leadsto discomfort for the occupants of the building, and in some cases itmay even be a health hazard. Examples of pollutants that often arepresent in indoor environments are gaseous odors, microorganisms andsmoke from tobacco.

Conventional treatment of polluted indoor air is focused on removal ofsolid particles by filtration, adsorption or electrostatic techniques.However, such removal only transfers the pollutants from one space toanother, and eventually the cleaning surface or the like is saturated,whereby the cleaning efficiency is lowered. Furthermore, low moleculargaseous components are difficult to remove with such techniques.

Alternatively, polluted air can be cleaned by use of photocatalyticair-cleaners. Polluted air is then passed over a photocatalytic surface,which is irradiated with light. Pollutants that are adsorbed on thephotocatalytic surface are then decomposed into harmless, odorless, andless toxic compounds. The process of photocatalysis is well known in theart, and no detailed description is given herein. One well-knownphotocatalyst is titanium dioxide (TiO₂), and other known photocatalystsare ZnO, CdS, WO₃, SnO₂, ZrO₂, Sb₂O₄, CeO₂ and Fe₂O₃. In general, theresulting photocatalytic activity of these materials is higher when theirradiated light is of high energy (short wavelength), such as light inthe ultraviolet spectrum. WO 96/37291, U.S. Pat. Nos. 5,045,288 and4,892,712 all show air cleaners utilizing photocatalysts.

Solar irradiation contains ultraviolet radiation, whereby it may be usedas irradiation source in a photocatalytic air cleaning system orpollutant decomposition device. EP 0590 477A1 discloses an example of anair cleaning system in the form of a window of double pane type, whereinthe air is passed in-between the two window panes and wherein aphotocatalyst is arranged in the intermediate space or on one or both ofthe intermediate window surfaces. However, such existing systems sufferfrom a number of disadvantages, which significantly lower the cleaningefficiency of the window, which are overcome by the present invention.

SUMMARY OF THE INVENTION

The object of the invention is to provide a new pollutant decompositiondevice, which overcomes the drawbacks of the prior art. This is achievedby the device as defined in claim 1.

One advantage with the pollutant decomposition device according to theinvention is that the cleaning-capacity of the device is significantlyincreased, while preserving over all optical properties.

Another advantage is that photocatalysts that require irradiation in theultraviolet spectrum to be activated can be used in pollutantdecomposition devices of this type.

Embodiments of the invention are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail below with reference to thedrawings, in which:

FIG. 1 shows examples of absorption spectra for photocatalytic TiO₂ oftwo different structures, anatase and rutile.

FIGS. 2 a and 2 b show one embodiment of a pollutant decompositiondevice according to the present invention.

FIG. 3 shows the ultraviolet transmittance for a standard glass and alow Fe₂O₃ glass.

FIG. 4 shows an example of the spectral solar irradiance at groundlevel.

FIGS. 5 a and 5 b show a section of another embodiment of a pollutantdecomposition device according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The pollutant decomposition device according to the present inventiongenerally relates to a solar radiation activated photocatalyticpollutant decomposition device, i.e. the irradiation that give rise tothe photocatalytic activity is mainly provided by the sun. However,during night time or at locations where sunlight is not present atsufficient amounts, the pollutant decomposition device may be irradiatedwith light comprising light in the ultraviolet spectrum from anothersource of light. Furthermore, the pollutant decomposition deviceaccording to the present invention comprises at least one sheet thatserves to fully or partly enclose the air or gaseous composition that isto be depolluted, and at least one sheet that is provided with aphotocatalyst. The pollutant decomposition device may e.g. be an aircleaning window of the type presented in EP 0590 477 wherein the outersheet is represented by the outer window pane, which together with thewalls etc. of the building encloses air to be depolluted.

As mentioned above, maximum photocatalytic activity of the preferredphotocatalysts is generally obtained when the photocatalyst isirradiated with light in the ultraviolet spectrum, i.e. light ofwavelengths less than approximately 380 nm. FIG. 1 shows examples ofabsorption spectra for photocatalytic TiO₂ of two different structures,anatase and rutile. As can be seen in FIG. 1, the lower absorption limitis about 370 nm for anatase and 400 nm for rutile. Therefore, it is ofgreat importance that the solar irradiation that reaches thephotocatalytic surface in a pollutant decomposition device comprises asmuch ultraviolet irradiation as possible, to achieve optimum efficiencyfor the decomposition process. As will be discussed in detail below,this may be achieved by selecting a material with low ultravioletabsorption for the outer layer in the pollutant decomposition device.However, in many applications, it is highly undesirable that suchultraviolet irradiation is permitted to pass through the device (inlarge amounts). Especially when the transmitted light enters theinterior of a building, as it then would induce degradation of organicmaterials, bleach textiles etc. Therefore the inner transparent sheetshould have a normal or high degree of ultraviolet absorption.

According to one embodiment of the pollutant decomposition deviceaccording to the present invention it is provided in the form of an aircleaning window (FIG. 2). Below, the air cleaning window according tothis embodiment is described as a window in a building, but it should beunderstood that the air cleaning window according to the invention maybe used in any window application, such as in automobiles, airplanes,boats and the like.

FIG. 2 shows one embodiment of an air cleaning window 10 according tothe present invention. An outer transparent sheet 20 and an innertransparent sheet 30 are arranged in a double glazing structure, suchthat an air gap 40 is formed between them. The outer transparent sheet20 has a non air gap surface 50 facing a source of irradiation, and anair gap surface 60 facing the air gap 40. Consequently, the innertransparent sheet 30 has a non air gap surface 70 facing away from thesource of irradiation, and an air gap surface 80 facing the air gap 40.A photocatalyst 90, which is activated by ultraviolet irradiation, isarranged in the air gap 40 to perform the air cleaning. To achievecirculation of air through the air gap 40, air conduits or holes 100 arearranged at the top and the bottom of the window 10, respectively.

As mentioned above, the outer transparent sheet 20 must show a highdegree of ultraviolet transmittance to achieve optimum cleaningefficiency. Therefore, the outer transparent sheet 20 is preferably madeof glass with a low Fe₂O₃ content, as the transmittance of ultravioletlight is highly dependent on the Fe₂O₃ content. FIG. 3 shows acomparison between a standard glass with a normal Fe₂O₃ content and alow Fe₂O₃ glass. The difference in transmittance in the ultravioletspectra may not seem very large, but experiments have shown dramaticincreases in reaction rates at the photocatalyst 90. The increasedreaction rates are clear when FIG. 3 is compared with the absorptionspectra in FIG. 1. FIG. 1 shows that the photocatalytic activity of TiO2increases significantly for wavelengths shorter than 350 nm, whereasFIG. 3 shows that the transmittance for standard glass dropssignificantly below 350 nm. Considering the solar spectrum shown in FIG.4 it can be seen that the irradiation intensity from the sun drops atwavelengths shorter than 350 nm, further enhancing the positive effectof the low Fe₂O₃ glass. More in detail, FIG. 3 shows that the ratio thatmay be obtained between the ultraviolet transmittance of the low Fe₂O₃glass and the standard glass is in the range of 10:8 to 10:1 or moredepending on wavelength. One example of a commercially available lowFe₂O₃ glass is Optiwhite by Pilkington.

As further mentioned above, the inner transparent sheet 30 shouldpreferably absorb a certain degree of ultraviolet irradiation.Therefore, the inner transparent sheet 30 is preferably made of astandard or a high Fe₂O₃ glass.

Preferably, the photocatalyst 90 is a thin, transparent film of TiO₂,but other suitable materials are listed above. In a preferredembodiment, the photocatalyst 90 is applied as a thin film on the airgap surface 80 of the inner transparent sheet 30, and one example of acommercially available glass of this type is Activ Glass by Pilkington.

To further enhance the cleaning process, the inner transparent sheet 30may have a low emission coating on the non air gap surface 70, wherebythe temperature of the inner transparent sheet 30 is raised accompaniedby increased reaction rates at the photocatalyst 90 and increased flowrate through the gap 40.

The air conduits or holes 100 may be formed in any suitable way, as longas sufficient circulation is achieved trough the air gap. The conduitsor holes 100 may further be arranged along the sides of the window oraround the perimeter of the same. Furthermore, forced circulation may beapplied by connecting a fan or the like to the air conduits or holes100.

Throughout the selection of window materials to optimize the cleaningaction of the window it is of great importance that the selections aremade such that the visual impression of the resulting window isacceptable.

To achieve thermal insulation where needed, one or both of the outer andthe inner transparent sheets 20, 30 could be replaced with an insulatingdouble-glazed window pane. The present invention is especially suitablefor buildings with large double-glazing areas.

According to another embodiment of the pollutant decomposition deviceaccording to the present invention, at least one of the outertransparent sheet and the inner transparent sheet is made of a flexibleplastic material, with a high degree of ultraviolet transmittance. FIGS.5 a and 5 b schematically shows one embodiment of this type wherein thepollutant decomposition device 110 is comprised of two sheet of flexibleplastic material 20 and 30 which are interconnected (not shown) so thata gap 40 is formed there between, and wherein air conduits in the formof perforations 100 are made in the inner transparent sheet 30 to createa flow of air in the gap 40. This embodiment may be used as a tarpaulinto provide an extremely versatile pollutant decomposition device. Ifdesigned so that the gap 40 between the sheets is self supported andthus preserved even when a slight pressure is applied on the layeredstructure, objects that are to be depoluted simply can be wrapped in theflexible pollutant decomposition device and hence be depolluted in anextremely effective manner. One possible way to make a self supportedgap 40 is to design gap forming interconnecting structures of inflatableelements, flexible foamed polymer material or the like.

In another embodiment of the pollutant decomposition device according tothe present invention, it is designed as a mobile depollution chamber,which may be used for depolluting polluted objects. Such mobiledepollution chambers and tarpaulin type depollution devices are veryuseful, e.g. at accident cites, in situations of chemical warfare or thelike where it is of great interest to depollute objects with shortdelay, and preferably at the cite as transportation of polluted objectsmay be hazardous. In one special embodiment the depollution chamber isused to house a mobile hospital, whereby the amount of hazardous germs,viruses in the air may be drastically lowered. Such depollution chambersmay be designed as a two layer tent, wherein a separate innertransparent sheet of flexible plastic material and a separate outertransparent sheet of flexible plastic material forms the two layers ofthe tent, and wherein openings suitably are arranged in the innertransparent sheet to provide an airflow in the gap between the twosheets. Alternatively, such chambers may be formed using theinterconnected tarpaulin type pollutant decomposition device of above.In one special embodiment, the inner transparent sheet of flexibleplastic material is divided into a large number of separate sheetsforming passages for the enclosed air there between.

An example of a flexible plastic material, with a high degree ofultraviolet transmittance is ethyl tetra fluoro ethylene. An example ofa flexible plastic material, with a low degree of ultraviolettransmittance is polyester. The applicants have successfully depositedphotocatalyst films on thin polyester substrates. To further enhance thestability of the polyester film, which may be degraded by uv-radiation,it may further be provided with an additional uv-absorbing film that istransparent to visible light.

1. Pollutant decomposition device comprising at least one outertransparent sheet and at least one inner transparent sheet beingarranged such that a gap is formed between them and such that the gap isin communication with a surrounding gaseous composition on one side ofthe device such that the gaseous composition can pass through the gap,the device further comprising a photocatalyst arranged in the gap on thesurface of the inner transparent sheet, the photocatalyst being incontact with the gaseous composition within the gap at a same surface asbeing irradiated through said outer transparent sheet, the irradiatedsurface of the photocatalyst being in contact with the gaseouscomposition within the gap, wherein the outer transparent sheet has ahigh degree of ultraviolet transmittance compared with the innertransparent sheet, wherein the ratio between the ultraviolettransmittance of the outer transparent sheet and the inner transparentsheet is in the ranged of 10:8 to 10:1.
 2. Pollutant decompositiondevice according to claim 1, wherein the inner transparent sheet has alow emission coating on the non air gap surface.
 3. Pollutantdecomposition device according to claim 1, wherein the outer transparentsheet is made of a flexible plastic material.
 4. Pollutant decompositiondevice according to claim 3, wherein the inner transparent sheet is madeof a flexible plastic material.
 5. Pollutant decomposition deviceaccording to claim 4, wherein the two sheets are interconnected so thata gap is formed there between, and wherein air conduits in the form ofperforations are made in the inner transparent sheet to create a flow ofair in the gap.
 6. Pollutant decomposition device according to claim 1,wherein the outer transparent sheet is made of low Fe₂O₃ glass. 7.Pollutant decomposition device according to claim 1, wherein the innertransparent sheet is made of a flexible plastic material.